Method for detecting echo path changes in echo cancellers

ABSTRACT

A method for detecting echo path changes in an echo canceller using the statistics of the echo canceling behavior (i.e. signal and performance information), to distinguish between new line and double talk conditions. A moving counter is incremented or decremented based on monitored levels of ERL (Echo Return Loss), ERLE (Echo Return Loss Enhancement), noise and signal energies. When the counter reaches a predetermined threshold value indicative of sustained poor echo cancellation performance, a determination is made that there is a probable new line condition (i.e. echo path change). This echo path change information is then passed to the echo canceller to enable re-convergence.

FIELD OF THE INVENTION

The present invention relates generally to detection of echo pathchanges in echo cancellers, and more particularly to a method ofmonitoring echo canceling behavior, to distinguish between new line anddouble talk conditions, and thereby detect echo path changes in echocancellers.

BACKGROUND OF THE INVENTION

The signal path between two telephones, involving a call other than alocal one, requires amplification using a four-wire circuit. The costand cabling required discourages extending a four-wire circuit to asubscriber's premises from the local exchange. For this reason, thefour-wire trunk circuits are coupled to two-wire local circuits, using adevice called a hybrid.

Hybrid echo, the primary source of echo generated from thepublic-switched telephone network (PSTN) is created as voice signals aretransmitted across the network via the hybrid connection at thetwo-wire/four-wire PSTN conversion points.

Unfortunately, the hybrid is by nature a leaky device. As voice signalspass from the four-wire to the two-wire portion of the network, theenergy in the four-wire section is reflected back, creating an echo ofthe speech signal. Provided that the total round-trip delay occurswithin just a few milliseconds, the echo results in a user perceptionthat the call is ‘live’ by adding sidetone, thereby making a positivecontribution to the quality of the call.

In cases where the total network delay exceeds 36 ms, however, thepositive benefits disappear, and intrusive echo results. The actualamount of signal that is reflected back depends on how well the balancecircuit of the hybrid matches the two-wire line. In the vast majority ofcases, the match is poor, resulting in a considerable level of signalbeing reflected back.

The effective removal of hybrid echo is one key to maintaining andimproving perceived voice quality on a call. This has led to intensiveresearch into the area of echo cancellation, with the aim of providingsolutions that can reduce echo from hybrids. By employing the results ofthis research, the overall speech quality is improved significantly.

It is known in the art to employ adaptive filtering to address hybridecho cancellation. In an adaptive filter, the filter coefficients arebased, in part, on feedback of filter output. Normalized Least MeanSquare (NLMS) adaptive filtering is one method, popular in echocancellation, to address reflections in the telephony system.

In such echo cancellers, the coefficients of an adaptive filter convergeto a certain echo path. Under ideal conditions, a generally acceptableconvergence time requires that the echo canceller achieve 27 dB of ERLE(Echo Return Loss Enhancement) in 0.5 sec. Once the coefficients areconverged, the echo is canceled from the input signal. When the echopath changes (i.e. call transfer, conferencing), the echo canceller hasto quickly re-converge to the new echo path or else the echo will beperceived by the user. Detecting line changes is a difficult problemsince the echo resulting from a new hybrid in a changed echo path andthe echo generated by the old hybrid from the converged adaptive filtercan easily be confused as a double talk signal.

Prior art solutions to this problem may be found in U.S. Pat. No.6,035,034 (Trump, Tonu): Double talk and Echo Path Change Detection in aTelephony System, and U.S. Pat. No. 6,226,380 (Heping, Ding): Method ofDistinguishing Between Echo Path Change and Double Talk Conditions in anEcho Canceller.

SUMMARY OF THE INVENTION

According to the present invention, there is provided a method fordetecting echo path changes in an echo canceller that uses thestatistics of the echo canceling behavior (i.e. signal and performanceinformation), to distinguish between new line and double talkconditions. In terms of speech dynamics, double talk conditions arerelatively short in duration, whereas a new line condition remainsactive. Using a moving counter (referred to herein as an Echo PathChange Counter or EPC Counter), an evaluation is made of the probabilitythat the echo canceller behavior is responding to an echo path changeand not a double talk scenario. By monitoring the ERL (Echo ReturnLoss), ERLE (Echo Return Loss Enhancement), noise levels and signalenergies, the Echo Path Change Counter is incremented or decremented.When the counter reaches a predetermined threshold value indicative ofsustained poor echo performance, a determination is made that there is aprobable new line condition. This echo path change information is thenpassed to the echo canceller to enable re-convergence.

BRIEF DESCRIPTION OF THE DRAWINGS

An embodiment of the present invention will now be described, by way ofexample only, with reference to the attached Figures, wherein:

FIG. 1 is a schematic representation an echo canceller according to thepresent invention; and

FIG. 2 is a flowchart of a method of detecting echo path changes inoperation of the echo canceller of FIG. 1, according to the presentinvention.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 shows an adaptive echo canceller according to the prior art. Areference signal (Rin) is applied to an input of echo canceller 100 andto the echo path (i.e. a network echo path resulting from line impedancemismatch) as Rout. The echo path gives rise to an Echo Return Loss(ERL), which is a measure of the actual amount of reflected signal. Ahigh ERL indicates only a relatively small signal reflected back to thetalker, and vice versa. The echo canceller 100 models an estimation ofthe echo introduced by the echo path using the well known NLMS algorithm(although other adaptive algorithms may be used), and subtracts the echosignal from the Line Input Signal (Sin) which contains the undesirableecho, via a subtractor 110. Provided that the transfer function of themodel of the echo path provided by echo canceller 100 is identical tothe transfer function of the echo path, the error signal becomes zeroand the echo canceller 100 converges to the correct transfer function,resulting in perfect echo cancellation. Echo Return Loss Enhancement(ERLE) is given by the expected echo level subtraction, and is anindicator of the amount of echo removed by an echo canceller.

Echo Return Loss Enhancement (ERLE) The ERLE is defined as:ERLE(dB)=10log₁₀[Power(Sin)/Power(Ein)].

The echo path change detection algorithm of the preferred embodiment isset forth in FIG. 2. Once the echo canceller has converged, thealgorithm starts (step 200), whereupon the Echo Path Change Counter isintialized and initial ERL, ERLE, noise levels (NoiseLevelRin andNoiseLevelSin) and signal energies (Rin and Sin) are obtained.

In general, the algorithm according to the present invention monitorsERL and ERLE changes in order to distinguish between double-talk andecho path changes. The moving Echo Path Change counter is incremented ordecremented depending on the following conditions:

Area of Strong Divergence (a “Y” Branch from Step 202)

In this state, the algorithm determines with a degree of high confidencethat the echo canceller is diverged, and that occurrence of a linechange is highly probable. Two conditions are monitored:

a) The ERLE is less than the StrongDivergenceThreshold (typically set at−3 dB). This indicates that the echo canceller adds signal energy,instead of subtracting the echo signal;

b) The energy of error signal (ein) is also monitored to validate thatthe energy is above the noise level (NoiseLevelSin) calculated on theline input signal.

In this case, the Echo Path Change counter is incremented by afast_step_increment (—e.g. an increment of 4), at step 204.

Area of Slight Divergence (a “Y” Branch from Step 206)

In this state the algorithm can not determine with high confidence thata new line condition exist. Monitoring the ERLE is not conclusive aschanges in the ERLE could also be due to slight double talk or transienteffects. Therefore, other tests are performed. The following conditionsapply:

a) ERL_current is bigger than a minimum_expected_ERL (whereminimum_expected_ERL is the minimum expected ERL of 6 dB given in theG.165 ITU-T standard), indicating the absence of strong double talk;

b) ERLE is less than the SlightDivergenceThreshold (typically set at−1.1 dB) indicating that the echo canceller is adding some signal energyto the near-end+echo signal input (sin) instead of subtracting the echoOR ERLE is smaller than the minimum expected ERLE (value based on theinitial ERL) AND the difference between ERL current and ERL initial isbigger/smaller than EPC_GoodToBad_ERL_Threshold (e.g. −6dB)/EPC_BADtoGood_ERL_Threshold (e.g. 6 dB).

c) The input signal energies (reference (rin) and near-end signal+echo(sin)) are bigger than the respective noise levels. If the signals areclose to the noise level, the reliability of the above decisions alsodrops.

In this case, the Echo Path Change counter is incremented by aslow_step_increment (e.g. an increment of 1), at step 208.

Area of Strong Double Talk (a “Y” Branch from Step 210)

In this state, ERLE indicates that the echo canceller is not wellconverged, but the reference signal is close to the noise level, therebyindicating a double talk scenario. The following conditions apply:

a) The reference signal is less than the respective noise level;

b) The near-end+echo (sin) is greater than the respective noise level;

c) ERLE is less than the ERLEDoubleTalkThreshold (typically 1.5 dB).

In this case, the Echo Path Change counter is decremented byslow_step_decrement (e.g. a decrement of −1), at step 212.

Area of Strong Convergence (a “Y” Branch from Step 214)

In this state the algorithm determines with a degree of high confidencethat the echo canceller is converged and no echo path change hasoccurred. Two conditions are monitored:

a) ERLE indicates that the echo canceller is well converged;

b) The input signal energies (reference (rin) and near-end+echo (sin))are greater than their respective noise levels.

In this case, the Echo Path Change counter is decremented byfast_step_decrement (e.g. a decrement of 4), at step 216.

When the EchoPathChangeCounter reaches a Maximum threshold (e.g. 128),indicated by a “Y” branch from step 218, an EchoPathChange (step 220) isindicated and this information is then passed to the echo canceller(step 222).

It will be appreciated that, although embodiments of the invention havebeen described and illustrated in detail, various modifications andchanges may be made. Different implementations may be made by thosefamiliar with the art, without departing from the scope of theinvention.

1. A method of detecting an echo path chance in an echo canceller havingan adaptive filter, comprising: a) initializing an Echo Path ChangeCounter; b) monitoring signal and performance information from said echocanceller indicative of one of either echo canceller convergence ordivergence; c) decrementing said Echo Path Change Counter based on anindication of echo canceller convergence; d) incrementing said Echo PathChange Counter in response to an indication of echo cancellerdivergence; and e) indicating said echo Path change in the event saidEcho Path Change Counter exceeds a predetermined threshold wherein saidmonitoring of said signal and performance information includesdetermining whether ERLE is greater than a minimum expected ERLE value(Expected ERLE) and input signal energies (Rin and Sin) are greater thantheir respective noise levels, in which case an indication of strongconvergence is provided whereupon said Echo Path Change Counter isdecremented using a fast decrement.
 2. The method of claim 1, whereinsaid monitoring of said signal and performance information includesdetermining whether ERLE of the echo canceller is less than aStrongDivergencerhreshold and whether the energy of an error signal(ein) of the echo canceller is greater than the noise level on the lineinput signal to said echo canceller, in which case an indication ofstrong divergence is provided whereupon said Echo Path Change Counter isincremented using a fast increment.
 3. The method of claim 1, whereinsaid monitoring of said signal and performance information includesdetermining whether a current ERL (ERL_current) of the echo canceller isgreater than a minimum expected ERL value (minimum_expected_ERL);whether ERLE of said echo canceller is less than aSlightDivergenceThreshold or said ERLE is less than a minimum expectedERLE value (Expected ERLE) and the difference between the current ERL(ERL_current) and ERL initial is greater than aEPC_GoodToBad_ERL_Threshold or less than a EPC_BADtoGood_ERL_Threshold;whether input signal energies to the echo canceller (Rin and Sin) aregreater than respective noise levels, in which case an indication ofslight divergence is provided whereupon said Echo Path Change Counter isincremented using a slow increment.
 4. The method of claim 1, whereinsaid monitoring of said signal and performance information includesdetermining whether a reference signal (Rin) to the echo canceller isless than a respective noise level (NoiseLevelRin), whether a near-endinput signal (Sin) is greater than a respective noise level(NoiseLevelSin) and whether ERLE is less than a ERLEDoubleTalkThreshold,in which case an indication of double-talk is provided whereupon saidEcho Path Change Counter is decremented using a slow decrement. Themethod of claim 1, wherein said monitoring of said signal andperformance information includes determining whether ERLE is greaterthan a minimum expected ERLE value (Expected ERLE) and input signalenergies (Rin and Sin) are greater than their respective noise levels,in which case an indication of strong convergence is provided whereuponsaid Echo Path Change Counter is decremented using a fast decrement.